1. Field of the Invention
The invention relates to an automatic gain control (AGC) circuit and a method which adjust a received signal according to a start-up mode such that the received signal has its amplitude correspond to a baseband signal level determined from a bit error rate (BER) characteristics of a demodulator.
2. Description of the Background Art
Traditionally, the automatic gain control circuit and method have been known to be applicable to a receiver which receives a signal modulated by a modulation scheme such as a packet wireless transmission scheme, and to be able to adjust the received signal such that the received signal has its amplitude correspond to a baseband signal level determined from the BER characteristics of the demodulator.
As shown in FIG. 11, for example, the receiver 10 has a low-nose amplifier (LNA) 28 and a variable-gain amplifier (VGA) 30 for AGC-controlling a received signal 102 input from an antenna 12, which are interconnected as illustrated. I and Q component analog-to-digital (A/D) converters (ADCs) 32 and 34 then convert the AGC-controlled analog signal to a corresponding digital signal. In response to the digital signal, an automatic gain control circuit 700 automatically controls the gain such that the digital signal has its amplitude level expected by the baseband signal processor.
The low-noise amplifier and variable-gain amplifier 28 and 30 adjust the received signal 102 and provide them to the I and Q component A/D converters 32 and 34 in the form of I and Q components 106 and 108, respectively. The I and Q component A/D converters 32 and 34 then generate the I and Q component digital received signals 110 and 112, respectively, and provide them to the automatic gain control circuit 700.
The AGC circuit 700 has a power value operator 18 which operates the power of the digital received signals 110 and 112, to produce an input signal 114. The input signal 114 is representative of a power value determined by the expression (I2+Q2)1/2, for example, where I represents the I component digital received signal 110, and Q represents the Q component digital received signal 112.
The input signal 114 is provided to a scaling section 20 including an operator 36 and a multiplier 702. The operator 36 first subtracts a target value 146 from the input signal 114 to obtain a difference from the subtraction. The multiplier 702 then multiplies the difference by a scaling coefficient 712, and outputs a result 714 from the scaling. The multiplier 702 performs the same scaling process regardless of the sign of the input signal 114. When the scaling coefficient 712 is 2−1, for example, the input signal 114, when representing a value of 8, will cause a scaling result 714 of 4, and the input signal 114, when representing a value of −8, will cause a scaling result 714 of −4.
The AGC circuit 700 usually performs the AGC control with some modulation scheme assumed. The scaling section 20 is adapted, in order to prevent the AGC tracking error according to each of a plurality of modulation schemes, to make the scaling coefficient correspond to any one of the modulation schemes, or to fix, when adapted to two modulation schemes, for example, the scaling coefficient to a midpoint between two scaling coefficients respectively appropriate to the two modulation schemes.
The scaling result 714 is then provided to an adder 22. The adder 22 adds the scaling result 714 to the immediately preceding previous data 128 registered in a register 24. The addition result 30 is provided to a control-signal generating section 26.
The control-signal generating section 26 includes a register 46, a timing generator 48, a comparator 50, a selector 52, and an operator 54. The register 46 first stores the addition result 130. In response to an update-timing signal 132 from the timing generator 48, the register 46 outputs the result 130 to the comparator 50 and operator 54 as control data 134. The comparator 50 compares the control data 134 with a predetermined comparison value 136. The comparator 50 then provides the low-noise amplifier 28 and selector 52 with an LNA control signal 138 representing whether or not the control data 134 is less than the comparison value 136. In response to the LNA control signal 138, the selector 52 outputs either one of zero data 140 and the comparison value 136 to the operator 54 as selected data 142. The operator 54 subtracts the selected data 142 from the control data 134 to create a VGA control signal 144, and outputs it to the variable-gain amplifier 30.
In this way, the AGC circuit 700 can use a scaling coefficient adapted to the modulation scheme of the received signal to perform the low-noise amplification and variable-gain amplification on the received signal to accomplish automatic gain control.
U.S. patent application publication No. US 2004/0218576 A1 filed in the name of Imagawa et al., discloses a receiver which switches a selector switch according to a communication scheme, CDMA (Code Division Multiple Access) or TDMA (Time Division Multiple Access). The receiver thus selects an output signal from the variable-gain amplifier when receiving the CDMA signal, and selects an output signal from the second orthogonal mixer when receiving the TDMA signal. The second orthogonal mixer converts the output signal from the variable-gain amplifier to a corresponding baseband signal. The receiver can thus suitably adapt itself to any of a plurality of communication schemes without having to include any special offset-voltage eliminator.
In the AGC circuit 700 as shown in FIG. 11, however, regardless of what value the scaling section 20 sets the scaling coefficient 712 to, it is impossible to attain the maximum accuracy of the I and Q component A/D converters 32 and 34 appropriate for a specific modulation scheme.
The receiver taught in Imagawa et al., would, when adapted to a plurality of modulation schemes in the automatic gain controlling of the received signal, need to arrange a large number of circuits and wiring lines in a complex manner.
A type of received signal such as a continuous signal and a packet signal conventionally requires a longer AGC training period of time. The recent high-speed packet transmission has, however, a shorter training period of time for the purpose of its high-speed transmission feature, and thus requires AGC tracking in a shorter time period to adjust the output level in the I and Q component A/D converters. A smaller scaling coefficient can provide a higher speed AGC convergence, while it can also provide more AGC tracking errors. Particularly, in the case where the result from the control on the low-noise and variable-gain amplification changes dynamically, the automatic gain control may not converge but oscillate within a short time period.
FIG. 12, for example, shows how the result from the LVA and VGA control changes dynamically. As shown, when the baseband signal is set to the zero level and the received input level changes from −30 dBm to −20 dBm, the LNA control signal switches from its ON state to its OFF state to change the LNA gain value from 30 dB to 0 dB, and the VGA gain value changes from 0 dB to 20 dB. It can be understood that a larger gain change occurs during the transitional period of that changing point than other changing points.
In the receiver 10 as shown in FIG. 11, the radio-frequency (RF) circuit 14 including the low-noise amplifier 28 and variable-gain amplifier 30 processes the received signal 102 by allowing a first mixer, not shown, to perform a frequency conversion after the process in the low-noise amplifier 28 but before the process in the variable-gain amplifier 30. The low-noise amplifier 28 is switched between its ON and OFF states to prevent the distortion in the first mixer. Because the ON/OFF switching of the low-noise amplifier 28 varies the gain, however, a received signal with a level which varies the control of the low-noise amplifier 28 may increase the tracking error.
The baseband signal is calculated from the relationship: baseband signal=received input level+LNA gain value+VGA gain value. The LNA and VGA gain values need to be adjusted to keep the baseband signal constant within a supposed received-level range. Particularly, the baseband signal has its characteristics degraded by the distortion in the first mixer. It is difficult, however, to adjust the low-noise amplifier for no distortion.
The conventional AGC circuit needs to always perform the automatic gain control during receiving a signal, thereby consuming a large amount of current. Particularly, in the packet transmission, the automatic gain control is active even in a time period in which no signal is received, thereby increasing the current consumption.